Modelling and control of potable water chlorination.

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Abstract

In potable water preparation, chlorination is the last step before the potable water enters the
distribution network. Umgeni Water Wiggins Waterworks feeds the Southern areas of Durban.
A reservoir at this facility holds treated water before it enters the distribution network. To
ensure an adequate disinfection potential within the network, the free chlorine concentration in
the water leaving the reservoir at the Umgeni Water Wiggins Waterworks should be between
0.8 and 1.2 mg/L. The aim of this study was to develop an effective strategy to predict and
control the chlorine concentration at the exit of the reservoir. This control problem is made
difficult by the wide variations in flow and level in the reservoirs, together with reactive decay
of the chlorine concentration.
A Computational Fluid Dynamic study was undertaken to gain understanding of the physical
processes operating in the reservoir (FLUENT software). As this kind of modelling is not yet
applicable for real-time control, compartment models have been created to simulate the
behaviour of the reservoir as closely as possible, using the results of the fluid dynamic
simulation.
These compartment models were initially used in an extended Kalman filter (MATLAB
software). In a first step, they were used to estimate the kinetic factor for chlorine consumption
and in a second step, they predicted the chlorine concentration at the outlet of the reservoir. The
comparison between predictions and data, allowed the validation of the compartment models.
A predictive control strategy was developed using a Dynamic Matrix Controller, and tested offline
on the compartment models. The controller manipulated the chlorine concentration in the
inlet of the reservoir in order to control the chlorine concentration in the outlet of the reservoir.
Finally, the simplest compartment model was implemented on-line, using the Adroit SCADA
system of the plant, in the form of a Kalman filter to estimate the chlorine decay constant, as
well as a predictive model, using this continuously-updated decay parameter. The adaptive
Dynamic Matrix Controller using this model was able to control the outlet chlorine
concentration quite acceptably, and further improvements of the control performance are
expected from ongoing tuning.